Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. When inspecting specular or quasi-specular surfaces such as semiconductor wafers bright field (BF) and dark field (DF) modalities may be used, both to perform patterned wafer inspection and defect review. In BF inspection systems, collection optics are positioned such that the collection optics capture a substantial portion of the light specularly reflected by the surface under inspection. In DF inspection systems, the collection optics are positioned out of the path of the specularly reflected light such that the collection optics capture light scattered by objects on the surface being inspected such as microcircuit patterns or contaminants on the surfaces of wafers. Viable inspection systems, particularly BF inspection systems, require high radiance illumination and a high numerical aperture (NA) to maximize the defect sensitivity of the system.
Current wafer inspection systems typically employ illumination sources of deep ultraviolet (DUV) radiation with wavelengths as short as 260 nanometers with a high numerical aperture (NA). In general, the defect sensitivity of an inspection system is proportional to the wavelength of the illumination light divided by the NA of the objective. Without further improvement in NA, the overall defect sensitivity of current inspection tools is limited by the wavelength of the illumination source.
In some examples of BF inspection systems, illumination light may provided by an arc lamp. For example, electrode based, relatively high intensity discharge arc lamps are used in inspection systems. However, these light sources have a number of disadvantages. For example, electrode based, relatively high intensity discharge arc lamps have radiance limits and power limits due to electrostatic constraints on current density from the electrodes, the limited emissivity of gases as black body emitters, the relatively rapid erosion of electrodes made from refractory materials due to the presence of relatively large current densities at the cathodes, and the inability to control dopants (which can lower the operating temperature of the refractory cathodes) for relatively long periods of time at the required emission current.
To avoid the limitations of electrode based illumination sources, incoherent light sources pumped by a laser (e.g., laser sustained plasma) have been developed. Exemplary laser sustained plasma systems are described in U.S. Pat. No. 7,705,331 assigned to KLA-Tencor Corp., which is incorporated by reference as if fully set forth herein. Laser sustained plasmas are produced in high pressure bulbs surrounded by a working gas at lower temperature than the laser plasma. Substantial radiance improvements are obtained with laser sustained plasmas. Atomic and ionic emission in these plasmas generates wavelengths in all spectral regions, including shorter than 200 nm when using either continuous wavelength or pulsed pump sources. Excimer emission can also be arranged in laser sustained plasmas for wavelength emission at 171 nm (e.g., xenon excimer emission). Hence, a simple gas mixture in a high pressure bulb is able to sustain wavelength coverage at deep ultraviolet (DUV) wavelengths with sufficient radiance and average power to support high throughput, high resolution BF wafer inspection.
Development of laser sustained plasmas has been hampered by reliability issues related to degradation of the bulb containing the gas mixture. Traditional plasma bulbs of laser sustained light sources are formed from fused silica glass. Fused silica glass absorbs light at wavelengths shorter than approximately 170 nm. The absorption of light at these small wavelengths leads to rapid damage of the plasma bulb, which in turn reduces optical transmission of light in the 190-260 nm range. In some examples, substantial emission of radiation in the vacuum ultraviolet range (VUV) causes the bulb material to degrade. VUV light with photon energies in excess of 6.5 eV (˜190 nm) causes rapid damage to materials used to construct the LSP lamphouse, and most importantly, to the material of the bulb itself. Fused silica glass undergoes rapid solarization, transmission loss, compaction-rarefaction and related stress, micro-channeling, and other damage that leads to reduced source output, loss of structural integrity (e.g., explosions), overheating, melting, and other adverse results.
FIG. 1 is illustrative of a plot 10 depicting the percentage of plasma emission absorbed by the bulb wall absorption as a function of wavelength for various bulb configurations and operating scenarios. Plotline 15 illustrates the absorption of an unexposed bulb. Plotline 14 illustrates a bulb containing Xenon gas after operation for one hour at five kilowatts output power, five hours at four kilowatts output power, and less than one hour at three kilowatts output power. Plotline 13 illustrates a bulb containing Krypton gas after operation for seven hours at four kilowatts output power. Plotline 12 illustrates a bulb containing Argon gas after operation for less than one hour at three kilowatts output power. Plotline 11 illustrates a bulb containing Krypton gas after operation for one hour at three kilowatts output power and two hours at four kilowatts output power. As illustrated in plot 10, only a few hours of operation results in significant absorption losses, particularly in the wavelength range between 200 nanometers and 260 nanometers.
In some examples, VUV-absorptive coatings are used to block VUV in ozone-free bulbs. The material composition of the coating determines the absorption profile of the coating. For a LSP to be an effective illumination source for inspection, an absorptive coating should not block light with wavelengths longer than 190 nm (DUV light) and absorb light with wavelengths shorter than 190 nm (VUV light). In this manner, shorter wavelength VUV light that causes damage to the bulb is absorbed without absorbing DUV radiation that is desired for inspection. Unfortunately, existing materials do not have a sharp absorption cutoff near 190 nanometers. Existing coating materials either absorb light in a desirable illumination range from 190-260 nanometers, or transmit substantial amounts of light with wavelengths shorter that 190 nm. Similar problems are encountered by trying to match the absorption edge of the coatings to radiation in the band between 260-450 nanometers. Moreover, the protective coating itself is subject to damage and early failure from exposure to VUV light.
As inspection systems with laser sustained plasma illumination sources are developed, reliability becomes a limiting factor in maintaining system uptime. Thus, improved methods and systems for extending the lifetime of laser sustained plasma sources are desired.